One of the most significant developments in semiconductor technology in recent years has been the increased use of III-V materials such as gallium arsenide and indium phosphide, and their ternary and quaternary alloys such as indium-gallium-arsenide-phosphide, as the active material of semiconductor devices. The band gap characteristics of such materials typically make them candidates for optoelectronic and photonic applications such as lasers, light emitting diodes and photodetectors. For integrated circuit use, their high electron mobility often makes them preferable to the more commonly used semiconductor, silicon. Fabrication of such devices often requires epitaxial growth of one or more layers on a single-crystal substrate (epitaxial growth refers to a method of depositing a material on a substrate such that the crystal structure of the deposited material effectively constitutes an extension of the crystal structure of the substrate).
The three broad classes of methods for depositing by epitaxial growth are liquid phase epitaxy, vapor phase epitaxy and molecular beam epitaxy which, respectively, involves deposition from a liquid source, a vapor source and a molecular beam. A particularly promising form of vapor phase epitaxy is a method for depositing from a gas including a metalorganic compound; this process, known as metalorganic chemical vapor deposition (MOCVD), is described in a number of scientific publications including, "Metalorganic Chemical Vapor Deposition of III-V Semiconductor," M. J. Ludowise, Journal of Applied Physics, Vol. 58, No. 8, Oct. 15, 1985, pp. R31-R55, and the paper, "Metalorganic Chemical Vapor Deposition," P. Daniel Dapkus, American Review of Material Sciences, Annual Reviews, Inc., 1982, pp. 243-268. MOCVD processes make use of a reactor in which a heated substrate is exposed to a gaseous metalorganic compound containing one element of the epitaxial layer to be grown and a gaseous second compound containing another element of the desired epitaxial material. For example, to grow the III-V material gallium arsenide, one may use the metalorganic gas triethylgallium [(C.sub.2 H.sub.5).sub.3 Ga] as the gallium source and arsine (AsH.sub.3) as the source of the group V component, arsenic. The gas mixture is typically injected axially at the top of a vertically extending reactor in which the substrate is mounted on a susceptor that is heated by a radio-frequency coil. The gases are exhausted from a tube at the end of the reactor opposite the input end.
While MOCVD offers many recognized advantages over other forms of epitaxy, several problems remain in the formation of high quality devices. Chief among these is the problem of obtaining good uniformity of deposition along the upper surface of the substrate. Since the proper operation of devices such as semiconductor lasers requires several different epitaxial layers, each only a few microns thick, it can be appreciated that significant deviations of thickness uniformity may result in serious differences in the operation of such lasers. Moreover, use of such devices in systems requires a great deal of reproducibility in their production which cannot be achieved if a sufficient uniformity of deposited layer thicknesses is not obtained.